Here is an excerpt from a sticky in the Naturally Aspirated forum on this site, which I hope gives you some of the information you were looking for. Check out the rest of the FAQ for some general information in other performance areas for the 2.5i

"3) Exhaust
“Where can I get the boxer rumble that Subaru is so famous for?” the rumble that is infamous on WRX and STi’s (also the LGT) is due to the unequal exhaust length (UEL) headers. Unfortunately the 2.5i features equal length headers and thus does not emit the noted boxer rumble.

However, there are several aftermarket exhaust options available, though they will not give your car the boxer rumble sound, they can still change the exhaust note favorably. All axleback options for the LGT will bolt right up to the 2.5i. The SPTv2 CBE has also been successfully installed on a 2.5i as well. This link http://legacygt.com/forums/showthrea...on-130408.html contains a compilation of aftermarket exhausts for the 2.5i

Aftermarket UEL headers can be fit onto the 2.5i, but at the moment there are no plug and play solutions available. See this thread for more details on how to customize the header setup for the 2.5i http://legacygt.com/forums/showthrea...ei-153134.html"

I think that what the other members above are just simply trying to recommend to you is that if you are looking for a worthy investment towards performance, you shouldn't be looking at headers. If your mind is set on headers, just check out above.